Continuously variable transmissions (CVTs), which are mounted on vehicles such as automobiles and change a shift ratio of a rotational output of, for example, an engine, are provided with a variator (shift mechanism) which can steplessly change the shift ratio by a hydraulic control. In a known example of such a variator, a power shift member, such as a chain or a metal belt, is bridged between a primary pulley (drive-side pulley) and a secondary pulley (driven-side pulley) of which rotational shafts are in parallel with each other, and changes the shift ratio by hydraulically changing an effective diameter of each pulley. In such a variator, for example, it has been known that the variator is configured to perform an upshift by filling a chamber provided in the primary pulley with hydraulic fluid and perform a downshift by discharging the hydraulic fluid from the chamber, and to be provided with an upshift control valve for switching a filling state of the hydraulic fluid to the chamber and a downshift control valve for switching a discharging state of the hydraulic fluid from the chamber. The upshift control valve and the downshift control valve are driven by respective shift control actuators such as solenoids.
Thus, in the continuously variable shift which changes the shift ratio using the actuators for upshifting and downshifting, appropriate diagnoses of normality and abnormality of each actuator is required. As a conventional technique relevant to such a diagnosis of the shift control actuator, for example, Japanese Unexamined Patent Application Publication (JP-A) 2006-248371 discloses that, when an upshift or a downshift of the continuously variable shift is not properly performed, upshift and downshift solenoids are turned on simultaneously, and which solenoid is failed is determined based on transitions of shift ratios at that time.
However, in the above conventional technique, because a particular state where both the upshift and downshift solenoids are turned on must be set after it is first determined that the shift control is not properly performed, the accomplishment to diagnose a failure takes a long period of time. Further, for example, in the case where the downshift solenoid becomes in an ON-keeping state, the hydraulic fluid is discharged at a maximum speed when the upshift solenoid is turned off. However, when the shift ratio becomes higher than a target shift ratio, the upshift solenoid is turned on by a feedback control and, thus, it is possible to fill the hydraulic fluid to the chamber. For this reason, hunching in which an overshoot is repeated occurs, but the feedback control itself which brings an actual shift ratio close to the target shift ratio is still possible, whereby the failure determination may not be performed.